Method for forming semiconductor device isolating regions

A method for forming semiconductor device isolation regions comprising steps of:

• forming a first insulating film on a semiconductor substrate,
• removing the first insulating film in portions to become device isolation regions with use of a resist pattern formed in an one-time lithography step as a mask so as to form a plurality of openings for forming the device isolation regions down to the semiconductor substrate,
• removing the resist pattern to deposit on the first insulating film and inside the opening a second insulating film of a stacked structure having a SiN film as the lowermost layer to a thickness more than half the width of an opening for the narrowest device isolation region, and then etching the entire surface in order to make the second insulating film remain on only the periphery of the bottom of the openings other than the opening for the narrowest device isolation region and to expose the surface of the semiconductor substrate in a central portion of the bottom of the openings, while the second insulating film remains so as to cover at least the opening for the narrowest device isolation region,
• forming an oxide film on the exposed surface of semiconductor substrate in the central portion of the bottom of the openings, except the opening for the narrowest device isolation region, with the first insulating film and the second insulating film remaining on the periphery of the bottom of the opening, acting as a mask by a selective oxidation method,
• removing the second insulating film remaining on the surface of the substrate and on the periphery of the bottom of the opening,
• etching the surface of the semiconductor substrate exposed on the periphery of the bottom of the openings with use of the oxide film formed in the central portion of the bottom of the openings by said selective oxidation method and the first insulating film remaining in portions other than the device isolation region as a mask so as to form trences, and at the same time etching the surface of the semiconductor substrate exposed in the bottom of the opening for the narrowest device isolation region so as to form a trench, and
• burying each trench with a third insulating film.